1. Field of Invention
The present invention relates generally to a method and apparatus for producing and delivering solvent vapor to the interior of vessels such as ship storage compartments, tank trucks, rail cars, land tanks and the like, so that residue deposits and/or protective coatings on the interior surfaces thereof or objects contained therein are exposed to solvent vapor for the purpose of chemically assisted cleaning and removal.
2. Brief Description of the Prior Art
Now, more than ever, a great need exists to remove residual and protective coatings from the interior surfaces of vessels, such as ship storage compartments, tank trucks, rail cars, land tanks and the like in an environmentally safe, cost-effective manner. Examples of such residue deposits and coatings include oil, grease, crude petroleum products, petroleum asphalt, coal tar products, resinous products, paints, plasticizers, epoxy and the like.
In the past, several different approaches have been used for cleaning these residues and coatings from the interior surfaces of large industrial vessels. For example, U.S. Pat. No. 4,530,131 to Zell, et al. discloses one approach to removing resinous deposits and/or coatings, such as oil and grease, from the interior surfaces of ship storage compartments and bilges. As disclosed, this approach involves the use of cleaning agents such as steam, hot water, detergents and solvents. Generally, these cleaning agents are applied using steam hoses, pressure wands, or rotating spray heads.
Although widely used, this prior art method suffers from a number of significant shortcomings and drawbacks. The required steaming, washing and flushing operations generate large quantities of waste water effluents and air emissions. As these effluents and emissions contain organic and inorganic pollutants, pretreatment processing is required prior to discharge to the environment. This processing involves complicated equipment and enormous time and labor, resulting in high costs. Moreover, since hardened or crystallized material remains on the interior surfaces of the vessel after the cleaning process is completed, chipping, scraping and/or grit blasting operations are also frequently required, resulting in injury to the surface being cleaned, additional expense and further waste disposal concerns.
An alternative approach utilizes suitable hydrocarbon solvents in the vapor state to remove residue deposits and/or protective coatings. Various types of solvents suitable for use in this approach are generally disclosed in column 4 of U.S. Pat. No. 4,357,175 to Buffington, et al. Prior art techniques and apparatus employing this approach can be found in U.S. Pat. Nos. 3,042,553 and 3,076,163 to Kearney, et al., and U.S. Pat. Nos. 4,303,454, 4,231,805 and 4,231,804 to Petterson, et al. The method disclosed in each of these references calls for a chlorinated hydrocarbon solvent such as methylene chloride to be converted to vapor which is then delivered to the interior of the vessel where it contacts the residue deposit and/or protective coating thereby cleaning the same from the interior surfaces. These references propose several types of apparatus for producing and delivering the solvent vapor to vessel interiors.
For example, the apparatus proposed by Kearney et al. employs an evaporation tank which produces solvent vapor that is transported to the vessel interior under high pressure. During vapor cleaning operations, solvent vapor within the vessel interior is withdrawn by a motor driven blower, subsequently condensed to liquid solvent in a condenser, and then returned to the evaporation tank for reuse. Solvent vapor which condenses on the wall surfaces of the vessel interior collects at the bottom thereof and is pumped out during the vapor cleaning operation and returned to the evaporation tank for distillation therein.
A method proposed by Petterson, et al., employs an evaporation tank in which solvent is delivered at about ambient temperature. The method further calls for recirculation of the vapor as a means of increasing vapor concentration within the vessel.
A prior art apparatus utilizing the vapor recirculation disclosed by Petterson has been proposed in which a rectangular shaped evaporator tank is integrally formed with a condenser unit extending from one wall surface, to selectively permit condensing of solvent vapor circulating within the evaporation tank and subsequent collection of solvent condensate for reuse. Through another wall of the evaporator tank opposite the condenser unit, a first port is provided for withdrawal of solvent vapor from the evaporation tank and delivery thereof to the vessel interior using a first vapor tube and a blower unit. Through another wall of the evaporator tank adjacent the condenser unit, a second port is provided for returning solvent vapor from the vessel interior through a second vapor tube.
While this prior art evaporation tank is capable of delivering solvent vapor to the interior of a vessel to be cleaned, the configuration and design of such apparatus is characterized by low vapor flow rates and low heat transfer between the evaporation chamber and the vessel interior.
Consequently, heating of large volume vessel interiors (e.g., ship storage tanks) to high temperatures has not been achievable solely through the heat transfer afforded by the solvent vapor flow stream produced by this apparatus. In addition, maintenance of high solvent vapor concentrations within the vessel interior at such elevated temperatures has likewise been unachievable. The net effect of such limitations has been inefficient use of solvent and reduced efficacy of vessel cleaning operations.